1. Field of the Invention
Typical solid electrolytic capacitors are generally divided into such a solid electrolytic capacitor 100 as disclosed in JP-A-60-220922 and shown in FIG. 1, and such a solid electrolytic capacitor 200 with a safety fuse as disclosed in JP-A-2-105513 and shown in FIG. 2.
2. Description of the Related Art
The former solid electrolytic capacitor 100 includes a capacitor element 1 arranged between a pair of lead terminals 5 and 6. The capacitor element comprises a porous anode chip body 2 formed by compacting and sintering valve (valve action) metal powder, an anode wire 3 made of valve metal powder and fixedly connected to one end surface 2a of the anode chip body 2, and a cathode electrode film 4 formed on the anode chip body 2 via a dielectric film and a solid electrolyte layer. In setting the capacitor element 1, the anode wire 3 of the capacitor element 1 is connected to the anode lead terminal 5 by welding for example, whereas the cathode lead terminal 6 is electrically connected directly to the cathode electrode film 4 of the capacitor element 1. These components are sealed in a package 7 made of synthetic resin.
Similarly, the latter solid electrolytic capacitor 200 with a safety fuse includes a capacitor element 1 arranged between a pair of lead terminals 5 and 6. The capacitor element comprises a porous anode chip body 2 formed by compacting and sintering valve metal powder, an anode wire 3 made of valve metal powder and fixedly connected to one end surface 2a of the anode chip body 2, and a cathode electrode film 4 formed on the anode chip body 2 via a dielectric film and a solid electrolyte layer. In setting the capacitor element 1, the anode wire 3 of the capacitor element 1 is connected to the anode lead terminal 5 by welding for example. The cathode electrode film 4 of the capacitor element 1 is electrically connected to the cathode lead terminal 6 via a safety fuse wire M which melts and breaks due to overcurrent or temperature increase. These components are sealed in a package 9 made of synthetic resin.
Conventionally, a capacitor element for such solid electrolytic capacitors is manufactured by the following method.
Firstly, as shown in FIG. 3, valve metal powder such as tantalum is compacted into a porous anode chip body 2 so that an anode wire 3 made of valve metal such as tantalum projects from one end surface of the anode chip body 2 and then sintered. Subsequently, as shown in FIG. 4, the porous anode chip body 2 is immersed in a chemical solution A such as an aqueous solution of phosphoric acid with one end surface 2a of the anode chip body 2 oriented upward. In this state, anodization is performed by applying direct current across an electrode B in the chemical liquid A and the anode wire 3. As a result, a dielectric film 2b of tantalum pentoxide, for example, is formed on the surfaces of metal particles of the anode chip body 2. At that time, a dielectric film of tantalum pentoxide, for example, is formed also on a root portion of the anode wire 3 connected to the anode chip body 2.
Then, as shown in FIG. 5, the anode chip body 2 is immersed in an aqueous solution C of manganese nitrate with the end surface 2a of the anode chip body 2 oriented upward. After the aqueous solution of manganese nitrate C infiltrates into the anode chip body 2, the anode chip body is pulled out of the solution of manganese nitrate C and baked. These process steps are repeated a plurality of times. As a result, a solid electrolyte layer 4a of metal oxide such as manganese dioxide is formed on the dielectric film 2b of the anode chip body 2.
Subsequently, the cathode electrode film 4 comprising a graphite layer as a base layer and a metal layer of e.g. silver or nickel as an upper layer is formed over the solid electrolyte layer 4a on the surface of the anode chip body 2 except for the end surface 2a. 
When the solid electrolyte layer 4a of metal oxide such as manganese dioxide is formed in the above-described manufacturing process of a capacitor element, the solution of manganese nitrate C rises onto the surface of the root portion of the anode wire 3 connected to the anode chip body 2. Therefore, the solid electrolyte layer of manganese dioxide or the like is formed also at the root portion as connected to the solid electrolyte layer 4a of the anode chip body 2. Therefore, in assembling the capacitor element 1 into a complete solid electrolytic capacitor 100, 200, when the anode wire 3 is connected to an anode lead terminal 5 made of a metal plate by e.g. welding, the solid electrolyte layer formed on the root portion of the metal wire 3 may come into contact with the anode lead terminal 5, which may cause electrical short and often result in product failure.
Conventionally, therefore, before or after forming the dielectric film 2b of tantalum pentoxide by anodization, a ring member 8 made of a water-repellent synthetic resin such as fluoroplastic is attached around the root portion of the anode wire 3, as disclosed in JP-A-2000-348975 and shown in FIG. 6(a), or a coating 8xe2x80x2 as shown in FIG. 6(b) is formed by applying water-repellent synthetic resin dissolved in a solvent to the root portion followed by drying. Then, in such a state as shown in FIG. 6(a) or 6(b), the above-described formation of the solid electrolyte layer 4a by immersing in the aqueous solution of manganese nitrate, pulling out from the solution and baking is performed. In this method, the ring member 8 made of water-repellent synthetic resin or the coating 8xe2x80x2 prevents the aqueous solution of manganese nitrate from rising up to the root portion of the anode wire, and hence prevents a solid electrolyte layer from being continuously formed on the root portion of the anode wire 3. Thus, it is possible to reduce the possibility of product failure in assembling in to a completed solid electrolytic capacitor.
However, when the ring member 8 is attached around the root portion of the anode wire 3 as is in the former case (See FIG. 6(a)), a gap is inevitably defined between the lower surface of the ring member 8 and the end surface 2a of the anode chip body 2 due to the irregularity of the end surface 2a. Further, a gap is also defined between the outer circumferential surface of the anode wire 3 and the inner circumferential surface of the ring member 8.
The gap between the outer circumferential surface of the anode wire 3 and the inner circumferential surface of the ring member 8 is formed because the inner diameter of a through-hole of the ring member 8 is made larger than the diameter of the anode wire 3 for easily fitting the ring member 8 around the anode wire 3. Further, fin or the like formed in punching the ring member 8 from a plate material also inevitably causes the formation of the gap.
Therefore, as shown in FIG. 5, when the anode chip body 2 is immersed in a solid electrolyte forming solution such as an aqueous solution of manganese nitrate C or the like for forming the solid electrolyte layer 4a, the solid electrolyte forming solution such as manganese nitrate solution flows into the gap between the lower surface of the ring member 8 and the end surface 2a of the anode chip body 2 by capillary action. Then, the solid electrolyte forming solution passes through the gap between the inner circumferential surface of the ring member 8 and the outer circumferential surface of the anode wire 3 to reach the upper surface side of the ring member 8. Thus, the ring member 8 cannot prevent the rising of manganese nitrate solution perfectly. Since a solid electrolyte layer is formed also on the upper surface side of the ring member 8 of the anode wire 3 as connected to the solid electrolyte layer 4a on the anode chip body 2, the possibility of product failure in assembling into a completed solid electrolytic capacitor is still considerably high.
Moreover, for assembling into a complete solid electrolytic capacitor 100, 200, the anode wire 3 is fixedly connected to an anode lead terminal 5 made of a metal plate by welding for example, as shown in FIG. 7. Therefore, the neck dimensions from the end surface 2a to the anode lead terminal 5 needs to be increased by as much as the amount of the thickness T of the ring member 8. Therefore, in the case where the entire length L of the completed solid electrolytic capacitor 100, 200 is determined in advance, the provision of the ring member 8 makes it impossible to increase the length H of the anode chip body 2 and hence to increase the capacitance. In the case where the capacitance is determined in advance, the entire length L increases by as much as the thickness of the ring member 8, which leads to an increase in size and weight of the capacitor.
Contrary to this, in the latter method (See FIG. 6(b)) in which a coating 8xe2x80x2 is formed by applying and drying synthetic resin dissolved in a solvent, the coating 8xe2x80x2 can be formed closely to the end surface 2a of the anode chip body 2 and to the outer circumferential surface of the anode wire 3 without defining a gap. Therefore, it is possible to prevent the solid electrolyte forming solution from rising up to the root portion of the anode wire 3 when the anode chip body is immersed in the solvent and hence to prevent the solid electrolyte layer from being formed continuously to the upper side of the coating 8xe2x80x2 on the anode wire 3.
However, when the synthetic resin is applied as dissolved in a solvent, the synthetic resin dissolved in the solvent infiltrates deep into the porous structure of the anode chip body 2, where the solid electrolyte layer cannot be formed. Thus, the infiltration of the synthetic resin dissolved in the solvent increases the region where the solid electrolyte layer cannot be formed. In other words, the effective volume of the anode chip body capable of functioning as a capacitor decreases, which results in a decrease of the capacitance. Further, the cost increases largely, because the application of the synthetic resin dissolved in a solvent to the entire circumference of the anode wire is very troublesome. Moreover, as shown in FIG. 8, the projecting height Txe2x80x2 of the coating 8xe2x80x2 from the end surface 2a of the anode chip body 2 becomes large. Therefore, in assembling into a completed solid electrolytic capacitor 100, 200 shown in FIGS. 1 and 2, the neck dimension S from the end surface 2a of the anode chip body 2 to the anode lead terminal 5 becomes further larger than the case where the ring member 8 is provided. Therefore, an increase in the capacitance of the capacitor is further hindered, and the size and weight is further increased.
Recently, as disclosed in JP-A-7-320983 for example and as shown in FIG. 9, the end surface 2a of the anode chip body 2 is formed with a recess 2c surrounding the anode wire 3, and a water-repellent synthetic resin P as dissolved in a solvent is loaded in the recess.
With such a method, it is possible to reliably prevent the solid electrolyte layer from being formed at the root portion of the anode wire and hence to decrease the neck dimension S in assembling into a completed solid electrolytic capacitor 100, 200 shown in FIGS. 1 and 2. However, since the synthetic resin as dissolved in a solvent is loaded in the recess 2c at the end surface 2a of the anode chip body 2, the synthetic resin infiltrates deep into the porous structure of the anode chip body 2 before it dries. Thus, a decrease in capacitance of the capacitor due to the infiltration of the synthetic resin cannot be avoided.
To attain these technical objects, a capacitor element according to a first aspect of the present invention comprises an anode chip body formed by sintering valve metal powder, an anode wire projecting from an end surface of the anode chip body, and a ring member made of a water-repellent thermoplastic synthetic resin and fitted around a root portion of the anode wire connected to the anode chip body, wherein the ring member is thermally melted in the fitted state around the anode wire.
Further, a process of making a capacitor element according to a first aspect of the present invention comprises the steps of fitting a ring member made of a water-repellent thermoplastic synthetic resin around a root portion of an anode wire connected to an end surface of an anode chip body formed by sintering valve metal powder, thermally melting the ring member fitted around the anode wire, forming a dielectric film on the anode chip body by anodization performed by immersing the anode chip body in a chemical solution, and forming a solid electrolyte layer on the anode chip body by immersing in and pulling out the anode chip body with respect to a solid electrolyte forming solution followed by baking.
A solid electrolytic capacitor according to a first aspect of the present invention comprises a capacitor element disposed between an anode lead terminal and a cathode lead terminal, the capacitor element comprising an anode chip body made by sintering valve metal powder, an anode wire projecting from an end surface of the anode chip body, and a cathode electrode film formed on the anode chip body via a dielectric film and a solid electrolyte layer. The anode wire of the capacitor element is connected to the anode lead terminal whereas the cathode electrode film is electrically connected to the cathode lead terminal. The anode wire has a root portion which is connected to the anode chip body and around which a ring member made of a water-repellent thermoplastic synthetic resin is fitted, and the ring member is thermally melted in its fitted state around the anode wire.
In this way, the ring member mounted to the root portion of the anode wire connected to the anode chip body is heated and melted in the state fitted around the anode wire. As a result, the ring member deforms into a configuration fitting to the end surface of the anode chip body and adheres closely to the end surface without a gap as if thermally fused to the surface while avoiding or considerably reducing filtration into the porous structure of the anode chip body.
Further, the fin produced in punching the ring member from the material plate member disappears by the thermal melting, and the ring member adheres closely also to the outer circumferential surface of the anode wire without a gap as if thermally fused to the surface while reducing the inner diameter of the through-hole.
According to the present invention, therefore, in forming a solid electrolyte layer on the anode chip body, the solid electrolyte forming solution can be reliably prevented from rising to the upper surface side of the ring member. In other words, it is possible to reliably prevent the formation of a solid electrolyte layer at a portion of the anode wire on the upper surface side of the ring member. Therefore, it is possible to greatly reduce the possibility of product failure due to electrical short between the anode lead terminal connected to the anode wire and the solid electrolyte layer without decreasing the capacitance of the capacitor.
Further, since the ring member adheres closely to both of the end surface of the anode chip body and the outer circumferential surface of the anode wire without a gap, it is possible to prevent the dielectric film from being formed at the portion of the end surface of the anode chip body provided with the ring member. Therefore, even when an external force for bending the anode wire is applied to the anode wire in handling as a capacitor element, the occurrence of dielectric breakdown at that portion can be reliably prevented. Thus, it is possible to greatly reduce defective products caused by dielectric breakdown occurring during the manufacturing process as a capacitor element.
Further, according to the present invention, the ring member is simply heated after fitted around the anode wire, and the process is very easy, so that an increase in cost can be suppressed to the minimum. Moreover, the height from the end surface of the anode chip body to the upper surface of the ring member can be suppressed so as not to exceed the initial thickness of the ring member, and variation in height can be reduced. Further, the neck dimension from the end surface of the anode chip body to the anode lead terminal can be decreased in connecting the anode lead terminal to the anode wire for assembling into a complete solid electrolytic capacitor. Therefore, it is possible to prevent an increase in size of the completed solid electrolytic capacitor or a decrease in capacitance of the solid electrolytic capacitor.
Particularly, as defined in claim 2, the ring member may be made of a transparent synthetic resin. In this case, it is possible to confirm, from the outside, whether or not the ring member adheres closely to both of the end surface of the anode chip body and the circumferential surface of the anode wire and whether or not the solid electrolyte forming solution enters between the ring member and the anode chip body or the anode wire in forming the solid electrolyte layer. Therefore, distinguishing between a proper product and a defective product can be easily performed.
Further, as defined in claim 3, the ring member may be formed with a cutout extending radially outward from a through-hole of the ring member to reach an outer circumferential surface of the ring member. With this structure, in fitting the ring member for attachment around the anode wire, the ring member can be fitted to a root portion of the anode wire from the side instead of passing the anode wire into the ring member like skewering. The cutout is filled and disappears when the ring member thermally melts, so that the ring member adheres closely to the entire circumference of the anode wire. Thus, the attachment of the ring member to the anode wire can be easily performed while keeping intended advantages.
Further, as defined in claim 9, the thermal melting of the ring member may be performed in vacuum or in an inert gas atmosphere. In this case, it is possible to reliably prevent or decrease the formation of an oxide film on metal particle surfaces of the anode chip body or on a surface of the anode wire and the change of properties of the dielectric film in thermally melting the ring member.
A capacitor element according to a second aspect of the present invention comprises an anode chip body formed by sintering valve metal powder, an anode wire projecting from an end surface of the anode chip body, and a ring member made of a water-repellent thermoplastic synthetic resin and fitted around a root portion of the anode wire connected to the anode chip body. The ring member has an inner diameter which is larger than the diameter of the anode wire so that the ring member adheres closely to the anode wire when the ring member shrinks due to heat. The ring member is thermally melted in contact with the end surface of the anode chip body.
A process of making a capacitor element according to a second aspect of the present invention comprises the steps of making an anode chip body of valve metal powder while connecting an anode wire to an end surface of the anode chip body, fitting a ring member made of a water-repellent and heat-shrinkable thermoplastic synthetic resin around the anode wire, the ring member having an inner diameter larger than a diameter of the anode wire so that the ring member closely adheres to the anode wire when the ring member thermally shrinks, thermally melting the ring member with the ring member held in contact with the end surface of the anode chip body, forming a dielectric film on the anode chip body by anodization performed by immersing the anode chip body in a chemical solution, and forming a solid electrolyte layer on the anode chip body by immersing in and pulling out of the anode chip body with respect to a. solid electrolyte forming solution followed by baking.
Further, a solid electrolytic capacitor according to a second aspect of the present invention comprises a capacitor element disposed between an anode lead terminal and a cathode lead terminal, the capacitor element comprising an anode chip body made by sintering valve metal powder, an anode wire projecting from an end surface of the anode chip body, and a cathode electrode film formed on the anode chip body via a dielectric film and a solid electrolyte layer. The anode wire of the capacitor element is connected to the anode lead terminal whereas the cathode electrode film is electrically connected to the cathode lead terminal. The anode wire is provided with a ring member made of a water-repellent thermoplastic synthetic resin and fitted around the anode wire, and the ring member has an inner diameter larger than the diameter of the anode wire so that the ring member adheres closely to the anode wire when the ring member thermally shrinks. The ring member is thermally melted in contact with the end surface of the anode chip body.
In this way, by thermally melting the ring member as held in contact with the end surface of the anode chip body, the ring member deforms into a configuration fitting to the irregular end surface of the anode chip body and adheres closely to the end surface without a gap as if thermally fused to the end surface while avoiding or considerably reducing filtration into the porous structure as compared with the case where a synthetic resin dissolved in a solvent is applied.
Further, since the ring member shrinks toward the anode wire by the thermal melting, the ring member adheres closely also to the outer circumferential surface of the anode wire without a gap as if thermally fused thereto. In this case, since the inner diameter of the ring member is larger than the diameter of the anode wire, it is possible to alleviate the rising of an inner peripheral portion of the ring member toward the upper surface side of the ring member due to the shrinkage toward the anode wire after having closely adhered to the anode wire. In other words, the height of the ring member from the end surface of the anode chip body can be reliably prevented from increasing due to thermal shrinkage.
According to the present invention, therefore, in forming a solid electrolyte layer on the anode chip body, the solid electrolyte forming solution can be reliably prevented from rising to the upper surface side of the ring member through a gap between the ring member and the anode chip body or the anode wire. In other words, it is possible to reliably prevent a solid electrolyte layer from being formed at a portion of the anode wire on the upper surface side of the ring member. Therefore, it is possible to greatly reduce the possibility of product failure due to electrical short between the anode lead terminal connected to the anode wire and the solid electrolyte layer in assembling into a complete solid electrolytic capacitor. At that time, the effective volume of the anode chip body capable of functioning as a capacitor is prevented from reducing.
Further, since the projecting height of the ring member from the end surface of the anode chip body can be decreased, the neck dimension from the end surface of the anode chip body to the anode lead terminal can be decreased in assembling the capacitor element into a complete solid electrolytic capacitor. Therefore, in the case where the entire length of the completed solid electrolytic capacitor is determined in advance, the length of the anode chip body can be increased by as much as the decreased amount of the neck dimension, thereby increasing the capacitance of the capacitor. In the case where the capacitance is determined in advance, the entire length of the solid electrolytic capacitor can be shortened by as much as the decreased amount of the neck dimension, thereby decreasing the size and weight of the capacitor.
Particularly, as defined in claim 5, the inner diameter of the ring member may be 1.20-1.60 times the diameter of the anode wire. In this case, it is possible to cause the ring member to closely adhere to both of the end surface of the anode chip body and the outer circumferential surface of the anode wire without a gap, while making the projecting height of the ring member from the end surface of the anode chip body smaller than the original thickness of the ring member. Thus, the above-described advantages can be further promoted.
A capacitor element according to a third aspect of the present invention comprises an anode chip body formed by sintering valve metal powder, an anode wire projecting from an end surface of the anode chip body, and a ring member made of a water-repellent thermoplastic synthetic resin and fitted around a root portion of the anode wire connected to the anode chip body. The end surface of the anode chip body is formed with a recess surrounding the anode wire, and at least part of the ring member is loaded in the recess by thermal melting.
Further, a process of making a capacitor element according to a third aspect of the present invention comprises the steps of making an anode chip body of valve metal powder having an end surface formed with a recess for surrounding an anode wire connected to the end surface, fitting a ring member made of a water-repellent thermoplastic synthetic resin around the anode wire, thermally melting the ring member so that at least part of the ring member is loaded in the recess, forming a dielectric film on the anode chip body by anodization performed by immersing the anode chip body in a chemical solution, and forming a solid electrolyte layer on the anode chip body by immersing in and pulling out of the anode chip body with respect to a solid electrolyte forming solution followed by baking.
Further, a solid electrolytic capacitor according to a third aspect of the present invention comprises a capacitor element disposed between an anode lead terminal and a cathode lead terminal, the capacitor element comprising an anode chip body made by sintering valve metal powder, an anode wire projecting from an end surface of the anode chip body, and a cathode electrode film formed on the anode chip body via a dielectric film and a solid electrolyte layer. The anode wire of the capacitor element is connected to the anode lead terminal whereas the cathode electrode film is electrically connected to the cathode lead terminal. The anode wire is provided with a ring member made of a water-repellent thermoplastic synthetic resin and fitted around the anode wire. The end surface of the anode chip body is formed with a recess surrounding the anode wire, and at least part of the ring member is loaded in the recess by thermal melting.
In the present invention, the recess provided at the end surface of the anode chip body is filled with at least part of the ring member thermally melted, not with synthetic resin dissolved in a solvent. Therefore, it is possible to cause the ring member to closely adhere to both of the anode chip body and the anode wire without a gap and as if thermally fused thereto while preventing or lessening infiltration of synthetic resin into the porous structure of the anode chip body.
Further, the projecting height of the ring member from the end surface of the anode chip body can be made smaller than the original thickness of the ring member by as much as the amount of the part of the ring member loaded in the recess.
According to the present invention, therefore, in forming a solid electrolyte layer on the anode chip body, the solid electrolyte forming solution can be reliably prevented from rising to the upper surface side of the ring member through a gap between the ring member and the anode chip body or the anode wire. In other words, it is possible to reliably prevent a solid electrolyte layer from being formed at a portion of the anode wire on the upper surface side of the ring member. Therefore, it is possible to greatly reduce the possibility of product failure due to electrical short between the anode lead terminal connected to the anode wire and the solid electrolyte layer in assembling the capacitor element into a complete solid electrolytic capacitor. At that time, the effective volume of the anode chip body capable of functioning as a capacitor is prevented from reducing.
Further, since the projecting height of the ring member from the end surface of the anode chip body can be decreased, the neck dimension from the end surface of the anode chip body to the anode lead terminal can be decreased in assembling the capacitor element into a complete solid electrolytic capacitor. Therefore, in the case where the entire length of the completed solid electrolytic capacitor is determined in advance, the length of the anode chip body can be increased by as much as the decreased amount of the neck dimension, thereby increasing the capacitance of the capacitor. In the case where the capacitance is determined in advance, the entire length of the solid electrolytic capacitor can be shortened by as much as the decreased amount of the neck dimension, thereby decreasing the size and weight of the capacitor.
Particularly, as defined in claim 7, the recess may have a depth which is generally equal to the thickness of the ring member. In this case, the ring member does not project from the end surface of the anode chip body almost at all so that the neck dimension can be further reduced, which promotes the above-described advantages.